In recent years, researches on fine fibers such as nanowires, nanotubes, and nanohorns have been actively made. As materials constituting nanowires, silver, silicon, gold, copper, zinc oxide, titanium oxide, gallium nitride, etc. have been studied. As nanotubes and nanohorns, for example, carbon nanotubes and carbon nanohorns are known, respectively.
Carbon nanotubes most promising as a conductive material have the structure of a graphite sheet rolled into a cylinder. Further, carbon nanotubes are a material having a hollow structure with a diameter of about 0.7 nm to 100 nm and a length of several micrometers to several millimeters. The electric properties of carbon nanotubes depend on their diameter or chirality, and range from metallic to semiconducting. Further, carbon nanotubes have no dangling bonds and are therefore chemically stable. Further, carbon nanotubes are composed of only carbon atoms, and are therefore attracting attention as an environmentally-friendly material.
Carbon nanotubes have such physical properties as described above, and are therefore expected to be used as an electron emission source for flat panel displays, an electrode material for lithium batteries, or an electrode material for electric double layer capacitors, or are expected to be applied to a probe.
Carbon nanotubes can be synthesized by, for example, arc discharge using carbon electrodes, thermal decomposition of benzene, or laser deposition. However, in these methods, impurities such as graphite or carbon nanoparticles are synthesized together with carbon nanotubes. Therefore, in order to apply the thus synthesized carbon nanotubes to the above-mentioned electron emission source, electrode for batteries, or probe, impurities such as graphite or carbon nanoparticles need to be previously removed. Further, the synthesized carbon nanotubes have various lengths and are oriented in random directions, and therefore their properties as an electron emission source are limited.
In recent years, a method for directly synthesizing oriented carbon nanotubes has been reported. For example, a method has been proposed, in which perpendicularly-oriented single-walled nanotubes are densely obtained on a Si wafer by plasma CVD. According to this method, the amount of impurities such as graphite or carbon nanoparticles is small, and carbon nanotubes whose fibers are oriented in the same direction can be obtained. Therefore, the produced carbon nanotubes can be easily applied to an electron emission source, an electrode for batteries, a probe, etc.
Further, active research and development have been carried out to apply carbon nanotubes to an electrode material (especially, an electrode material for electric double layer capacitors) by taking advantage of their large surface area. Further, there is an example using an electrode obtained by vertically growing carbon nanotubes on the surface of a current collector with the above-described technique.
An electric double layer capacitor is a condenser utilizing an electric double layer formed between an electrode active material and an electrolytic solution, and has been used as a backup power supply. However, such electric double layer capacitors have recently come to be used in electric cars, and are therefore expected to rapidly grow in the future. As a conventional electrode active material for electric double layer capacitors, one using activated carbon is widely known (see, for example, Patent Document 1). However, carbon nanotubes have an outer surface area of 2600 to 3000 m2/g, which is much larger than that of activated carbon, and exhibit very strong mechanical properties and excellent electronic properties. For this reason, electric double layer capacitors using carbon nanotubes as an electrode active material are attracting attention.
Electric charge Q stored in a condenser is generally expressed as Q=CV, where C is the capacitance of the condenser and V is voltage difference. Energy E stored in the condenser is expressed as E=½CV2. Therefore, stored energy per unit volume of active material is proportional to the capacitance of the condenser per unit volume of active material.
The stored energy of an electric double layer capacitor is preferably as large as possible. Therefore, methods have been developed to increase a condenser capacitance by subjecting an electrode active material to some kind of treatment.
Patent Document 2 discloses an electrochemical capacitor in which two or more electrodes composed of an active material powder, a conductive auxiliary agent (carbon powder), and an organic binder are opposed to each other with a separator interposed therebetween, and a space between each of the electrodes and the separator is filled with an aqueous electrolytic solution containing a quinone-based compound dissolved therein.
Patent Document 3 discloses the use of a carbon material/conductive polymer composite material, formed by coating the surface of a carbon material (carbon black or activated carbon) having a high specific surface area with a conductive polymer that causes a specific oxidation-reduction reaction, as an electrode for energy storage elements such as capacitors.
Patent Document 4 does not give a description about an increase in condenser capacitance, but discloses a technique to stabilize the dispersion of a carbon material, such as activated carbon, graphite, or conductive carbon, in a composition for capacitors by adding an organic dye derivative, an anthraquinone derivative, an acridone derivative, or a triazine derivative having a basic functional group or an organic dye derivative or a triazine derivative having an acid functional group to the composition for capacitors.
Patent Document 5 does not give a description about an electrode material, but discloses a method for purifying carbon nanotubes by irradiating the carbon nanotubes with ultraviolet light in an oxygen-containing atmosphere (air or oxygen atmosphere) to decompose and remove impurities.